A typical use of an insulation resistance tester is to test the insulation resistance of a capacitor. In an insulation resistance test, a good sample has a high insulation resistance value. However, a problem with conventional insulation resistance measuring apparatus is that such a contact check cannot be performed on the basis of measuring the insulation resistance value. In other words, if there is a loose contact between the tester device and a sample which has insufficient insulation, the tester may indicate that the sample has sufficient insulation. This problem becomes apparent in an automatic inspection process.
When a capacitor is used as a sample, since the capacitor represents a low impedance to an AC signal, the insulation resistance of the capacitor is measured by using a DC signal, while the high-frequency impedance thereof is measured by using an AC signal. On the basis of these measurements, it can be determined whether the measurement of the insulation resistance is valid, namely whether the contact is valid.
In general, an insulation resistance tester, as shown in FIG. 1, comprises a measuring device main frame 1 and an extension cable 5. The main frame 1 further comprises a shielded outer sheath 11 and a DC ammeter 12 having one input terminal connected to the shielded outer sheath 11, and a DC voltage source 13 for applying a DC high voltage between the outer sheath 11 and a ground. The extension cable 5 further comprises a first conductor 31 constituting a center conductor; a second conductor 32 for covering the first conductor, and a third conductor 33 for covering the second conductor 32. At one end of the above-described cable 5, the first conductor 31 is connected to the other input terminal of the DC ammeter 12, and the second conductor 32 is connected to the outer sheath 11. The third conductor 33 is grounded. A sample 6, shown as a parallel circuit comprising a capacitor C and a resistor R.sub.DC, is connected to the sample connecting terminals of the first and third conductors, 31 and 33, at the other end of the cable 5.
In an automatic inspection process, an impedance measurement is taken in conjunction with the above-described contact check in order to determine whether a sample demonstrates a predetermined characteristic. When there is a long distance between the sample 6 (namely, an object to be measured; the sample or object to be measured will be hereinafter referred to as a "DUT") and a measuring apparatus, the cable 5 from the measuring terminal is extended to, for instance, 1 to 2 meters for the impedance measurement. Also, in this setting, such a measurement must be taken while one of the input lines to the DUT is grounded. The following conditions are required for a successful measurement in the above described setting:
(1) An increase in capacitance should be detected when a DUT is connected to the measuring apparatus, even with an extended cable.
(2) A measurement of such a DUT should be taken with an AC grounded lead wire.
(3) The current path to measure a DC resistance is not adversely influenced by external electromagnetic fields and the like. Furthermore, conversely, this current path does not have any electromagnetic influence on an external device.
(4) An additional cable to measure impedance is not required.
Although the conventional impedance measuring apparatus, such as HP4192A: LF Tmpedance Analyzer sold by Hewlett Packard Co. in the U.S.A., can satisfy condition (1), such conventional measuring apparatus has the following problems: It is impossible to connect a DUT with one grounded lead wire in the measuring circuit with a high impedance because one of two lead wires of the DUT is driven by a measurement signal source and the other is virtually grounded through a current measuring device. Thus, the above-described condition (2) cannot be satisfied. When a ground terminal of a DUT is not grounded, an adverse influence over a measurement caused by an external electromagnetic field becomes conspicuous. This may considerable deteriorate a precision measurement so that the above-described condition (3) cannot be satisfied. In addition, while the insulation tester handles high DC voltage (1000 volts) and low DC current (less than pico amperes), the impedance meter handles AC voltage and current in a 10 Hz to 1 MHz range. These different operations make it difficult to combine the insulation tester and impedance meter.
The present invention has been proposed to solve the above-described problems. An object of the invention is to provide a two-terminal circuit element measuring apparatus for performing a contact check function for sensing a loose contact at a terminal so as to allow a high precision measurement by a single apparatus. A related object is to provide apparatus for measuring the impedance of a DUT with a high precision in connection with an automatic component feeding apparatus.